Travellers flying to the United States from Europe and the Middle East face tighter airport security checks from today, due to fears extremist Muslim groups are making new explosives able to slip by standard checks.

The measures threaten fresh disruptions for passengers at the start of the summer holiday season and will affect some direct flights to the US.

Officials declined to say if they had uncovered a specific plot.

But the move comes amid increased concern among Western intelligence services that Islamist fighters who go to Syria with European passports could pose a security risk on their return.